The presence of electrostatic charges and the subsequent discharge of the same can create significant problems in certain industries, particularly where explosive materials are involved and in electronic manufacturing or assembly plants. Individuals working in an everyday work environment commonly may develop thousands of volts of electrostatic charge potential by, as an example, walking across carpeting or moving dissimilar objects against each other. An individual, or object, so charged presents a severe hazard in the above identified environments. In an explosive materials environment the danger is inherently obvious. In the electronic integrated circuit (component) industry, the charged individual or object, may discharge near or through a static charge sensitive electronic component. For example, an individual who is electrostatically charged may hold a static charge sensitive component and then lay the component on a surface, e.g., a grounded work surface, at a different potential from the individual. At the instant of contact, a potential difference of thousands of volts exists across the component, from the electrostatically charged individual to the grounded work surface. The current passing through or near, due to the electric field generated, may damage the component. The damage caused to the component may cause it to fail immediately or, worse, could degrade the operating characteristics or the reliability of the component. The result is either expensive rework or, worse, the existence of substandard or subreliable equipment in the field.
A device which is used to help control the electrostatic charge buildup on a person is a body strap or wrist strap to be worn by the individual. The body straps are conductive on the surface contacting the skin surface and provide for an electrical connection point. An electrical ground cord may then be connected to the strap connecting the strap to an electrical ground potential, preferably through a predetermined limiting resistance usually built into the connector or the cord itself. So connected, such a body strap operates by draining any accumulated electrostatic charge on the individual to ground before the electrostatic charge buildup reaches dangerous levels.
One prior art body strap is described in U.S. Pat. No. 4,398,277, Christiansen et al, Conductive Elastomeric Body Strap. Christiansen et al describe a body strap which is constructed from a band of fabric formed into a closed loop to encircle a body part, e,.g., wrist, to which it is to be connected. The fabric is electrically conductive on the interior surface of the closed loop contacting the skin. A mechanical connection mechanism holds the loop of fabric in a fixed predetermined size. An electrical connection mechanism provides for an electrical connection between the conductive inner surface of the fabric to an electrical grounding cord which may be attached to the strap. The fabric is elastomeric to enable the body strap to expand to slip over the hand and still be snug around the wrist.
In the body strap described in Christiansen et al, the opposite ends of the fabric are permanently secured in the mechanical connector. The body of the connector has projections which grip the fabric and hold the fabric in the connector when the cover is secured. Thus, the resultant body strap formed is a fixed closed loop size. Since the fabric has a limit on the degree of its elastic nature, a range of sizes of closed loops for the body strap must be provided. This results in the necessity of stocking a plurality of differing sizes of body straps. Further, the elastomeric characteristics of the fabric generally means a fixed "life" of use of the fabric before its elastomeric or electroconductivity characteristics begin to break down. Since the fabric is secured in the connector at the factory, the replacement of the fabric requires replacement of the entire body strap.
The Charge-Guard 2200 series of static control wrist straps manufactured by Minnesota Mining and Manufacturing Company, St. Paul, Minnesota is constructed generally as described in Christiansen et al (Charge-Guard is a registered trademark of Minnesota Mining and Manufacturing Company, St. Paul, Minnesota). In the Charge-Guard static control wrist straps, the ends of the projections 32, in Christiansen et al, are sonically welded after the fabric is in place to "Mushroom" the ends of the projections in order to ensure that the fabric is secured in the connector.
U.S. Pat. No. 4,577,256, Breidegam, Woven Stretchable Grounding Strap, describes a wrist strap designed to be used to control electrostatic charge accumulations. The Breidegam strap has a clasp which allows its size to be adjusted. The adjustable clasp avoids the need to manufacture two or more models of the strap for different sized wrists. This does require that the strap be individually adjusted to fit snugly around the wrist of the individual wearer. If inadvertently or intentionally maladjusted, proper electrostatic protection may not be achieved. In the Breidegam strap, one end of the fabric is permanently secured into the clasp and held by plate and a rivet. Thus, one end of the fabric is fixed at the factory for the entire life of the strap. The second end of the fabric is engaged in the clasp by a pivotally mounted gate which when closed "jams" the fabric holding it in place, optionally with teeth to help the securing of the fabric. Typically, a pivotally mounted "jam" or "wedge" as is described in Breidegam is referred to as an "overcenter" device. These devices operate by wedging the fabric between the jam member and a reaction member by using an eccentric pivot with a relatively long jam operating lever to gain the necessary leverage for the jam to work. One problem in a strap as described in Breidegam is that it does not allow for full 360 degree electrical contact with the skin and the fabric is electrically connected only at one end. Since electrical contact is only provided to the external ground cord from the one fixed end of the fabric, any charge contacting the inner surface of the fabric must travel around the strap in one direction only until reaching the fixed end. This requires, in some instances, a charge to follow only one path to travel almost entirely around the fabric before being connected to a ground strap. Since the electrical conductivity of the fabric, for several reasons, is typically the weakest link in a wrist strap grounding system, along with the fabric to skin contact, such one way only conductivity is a serious problem. Another problem with the Breidegam strap is that the pivotally mounted nonconductive clasp does not lend itself to economical manufacture. Because of the forces involved, the pivot points are required to be quite sturdily built.
Earlier U.S. Pat. No. 4,845,585 discloses an adjustable wrist strap where the band is laid into a recess 36 in the molded part above the back plate 32. As the cover 42 was closed over the strap and a bar having spikes 40, the cover caused the strap to be forced into more intimate contact with back plate 32. The spikes 40 also held the strap from pulling free of the cover. This band affords 360 degree contact with the wrist.
Other prior art includes an earlier wrist band available from Simco Company, Inc, Hatfield, PA 19440, referred to as the Simco Trustat.TM. lay-in band which band had a strip of conductive material with one end terminated in an opening in a molded head, which had a slotted opening near one edge in the bottom wall and the conductive metal back plate had a folded edge which penetrated this slotted opening in the bottom wall and a serrated edge on the folded end of the metal plate pinched the end of the strip against the top wall of the head when a fastening rivet for the cord snap was placed through the metal back plate, the bottom of the head, the strip and the top surface of the head to engage the cord snap and hold the members together. This wrist strap also included a tail portion having a projecting lower plate with a raised bar transversely thereof which was positioned between two ribs on a hinged cover to place an S-bend in the strip and hold the other end of the strip to the body of the structure. There did not appear to be any contact between the second end of the strip and the metal back plate at the tail portion of the connector.
The present invention is a modification of the band of U.S. Pat. No. 4,845,585, an improvement in that it undoubtedly makes the 360 degree contact and electrical continuity between the band and the connector with the wrist.